This invention pertains to a rotating disc photosedimentometer useful in determining particle size and size distribution. In particular, this invention, by precisely controlling transmission of torque to the rotating disc, enables reproducible conditions needed for consistent and meaningful sample analysis. Heretofore, rotating disc photosedimentometers have been provided in which test conditions were subject to substantial variation and nonreproducibility due to the imprecise control of the electric motor applying torque to the rotating disc.
U.S. Pat. No. 3,475,968 discloses a method and apparatus for determining particle size and distribution, and also a method for using that apparatus to carry out the centrifugal analysis of particle size and size distribution. To conduct such an analysis by the rotating disc method, it is essential in one particular experimental method to establish a continuous density gradient in a liquid medium composed of two fluids, a spin fluid and a buffer fluid. Because the buffer fluid is less dense than the spin fluid and is injected after all the spin fluid, it is necessary to cause partial mixing of the fluids to establish the continuous density gradient. This partial mixing is accomplished by overriding the speed control of the electric motor, which in turn causes a rapid positive or negative acceleration of short duration in the rate of rotation of the disc centrifuge. The speed control is overridden by manually pushing a button for a specified amount of time. The period of time the button is pushed controls the length of motor control override and hence the period of partial mixing. Because the period of partial mixing varies from run to run, the continuous density gradient also varies. Hence, reproducibility of results is difficult to obtain.
There is therefore, a need for a rotating disc photosedimentometer which provides for precisely controlled and consistent periods of partial mixing to produce consistent continuous density gradients, and thereby assure reproducibility and reliability of results. Since these devices are of especial utility in control of latex production on a commercial scale, the need for such accuracy can be readily appreciated.